1. Field of the Invention
The present invention relates to a water purification system. More particularly, a water purification system in which a reject stream from a reverse osmosis unit is treated and used in a concentrating compartment of an electrodeionization unit is presented.
2. Description of the Related Art
Highly purified water having a small concentration of ions and other contaminants is required for a number of industrial applications. For example, highly purified water must be used in the manufacture of electronic microchips: mineral contaminants can induce defects. Highly purified water is used in the power generation industry to minimize the formation of scale on the interior of pipes and thereby ensure good heat transfer within and unrestricted water flow through heat exchange systems. The use of highly purified water reduces the formation of scale and deposits in water lines of heat exchange systems, thus extending the time interval between required maintenance procedures. The time interval between required maintenance procedures of a heat exchanging system should be as long as possible. Prolonging the time interval between required maintenance procedures is of particular importance in nuclear power systems, which require complex and expensive shutdown and startup procedures and adherence to radiation safety protocols.
Several technical approaches towards water purification exist, including the use of ion-exchange resins. However, the need to periodically regenerate ion-exchange resins requires a complex arrangement of pumps, piping, valves, and controls with associated large capital and maintenance costs and the use of regenerating chemicals which must be disposed of as chemical waste.
An alternative approach towards water purification is electrodialysis. An electrodialysis unit can include a positively charged anode, a negatively charged cathode, and alternating concentrating compartments and diluting compartments interposed between the anode and cathode. The electrical field established between the electrodes is understood to cause negatively charged anions to diffuse towards the anode and positively charged cations to diffuse towards the cathode. The concentrating compartments and diluting compartments are separated by compartment-separation ion-exchange membranes. An anion-exchange membrane bounds a diluting compartment on the side closer to the anode and allows anions to pass through while restraining the passage of cations. A cation-exchange membrane bounds a diluting compartment on the side closer to the cathode and allows cations to pass through while restraining the passage of anions. Direct electrical current is made to flow between the anode and the cathode to remove ions from the diluting compartments and concentrate ions in the concentrating compartments. A diluting feed stream of water can be continuously provided to the diluting compartments and a concentrating feed stream can be continuously provided to the concentrating compartments. The product stream flowing out of the diluting compartments is purified and contains a smaller concentration of ions than the diluting feed stream; the product stream can be further purified or can be provided to an industrial process for use. The concentrate effluent stream flowing out of the concentrating compartments contains a larger concentration of ions than the concentrating feed stream and can be recycled or discharged to a waste unit. An electrodialysis unit does not require the use of regenerating chemicals. Electrodialysis units are manufactured by Ionics, Incorporated of Watertown, Mass.
A water purification system should be energy efficient, i.e., should consume the least amount of energy per unit volume of purified water produced as is possible. Energy can be consumed, for example, in increasing the pressure of a supply stream of water in order to drive permeate through a membrane that filters out impurities, or in applying direct current across electrodes to drive ions into concentrating compartments in an electrodialysis unit. In an electrodialysis unit, it is understood that a large resistance, i.e., a small conductance, across the diluting compartment, the concentrating compartment, or both can result in a large fraction of the electrical energy supplied being dissipated as heat without driving the motion of many ions. This problem can be addressed in part by ensuring a large concentration of ions in the concentrating compartment by, for example, recycling the concentrate effluent stream to the entrance of the concentrating compartment or by adding salt to the concentrating feed stream.
The problem of small conductance across the diluting compartment is addressed with an electrodeionization unit. The basic design of an electrodeionization unit is similar to that of an electrodialysis unit. However, diluting compartments of an electrodeionization unit contain ion-exchange beads which increase conductance across the diluting compartment. The ion-exchange beads have positively and negatively charged sites; these sites facilitate the efficient migration of ions through the diluting compartment even when the conductivity of the diluting feed stream is low. An electrodeionization unit is capable of producing higher purity water than an electrodialysis unit.
Electrodeionization units can require periodic maintenance to clean or replace compartment separation membranes which have become fouled and through which the passage of ions has become impeded. Compartment separation membranes can become fouled through the deposition of scale formed from polyvalent ions such as Ca2+ and Mg2+ and counterions. Deposition of other impurities, such as bacteria, can also foul compartment separation membranes. Furthermore, although an electrodeionization unit can be effective at separating minerals from water, it may not efficiently remove other contaminants, such as organic carbon or bacteria. To address these problems, a filter for reducing the concentration of polyvalent ions and non-mineral impurities, such as organic carbon and bacteria, can be included upstream of the electrodeionization unit: the filter permeate stream can be provided as the diluting feed stream to the diluting compartment of the electrodeionization unit. A reverse osmosis unit with a reverse osmosis membrane can remove most bacteria, most organic carbon with a molecular weight greater than about 150 g/mol, and a large fraction of polyvalent ion impurities. Therefore, by providing a reverse osmosis permeate stream as the diluting feed stream, fouling of a compartment separation membrane from the side of the diluting compartment can be slowed or eliminated. The EDI product stream (electrodeionization product stream) exiting the diluting compartment can have a concentration of non-mineral impurities such as bacteria and organic carbon and a concentration of ions substantially reduced from the concentrations in a supply stream provided to the reverse osmosis unit.
As mentioned above, the concentrating feed stream should contain a large concentration of ions so that the conductance across the concentrating compartments is large. In one approach to ensure a large concentration of ions in the concentrating compartments, a water purification system incorporates a recycle pump which cycles the concentrate effluent stream exiting the concentrating compartment of the electrodeionization unit back to be used as the concentrating feed stream provided to the concentrating compartments. The subsystem including the recycle pump, piping connecting the recycle pump to the inlets and outlets of concentrating compartments, and concentrating compartments can be termed a concentrate loop. As ions are driven by the applied direct current from the diluting compartments into the concentrating compartments, the concentration of ions in the concentrate loop, including the concentrating compartments, increases. Eventually, a large concentration of ions in the concentrating compartments can result in a large conductance across the concentrating compartments. However, when the electrodeionization system is first started, there will only be a small concentration of ions in the concentrating compartments, and, therefore, only small conductivity of the fluid in the concentrating compartments and small conductance across the concentrating compartments. To increase the conductivity of the fluid in and the conductance across the concentrating compartments, salt as a source of ions can initially be injected into the concentrate loop.
Polyvalent ions driven from the diluting compartments into the concentrating compartments can accumulate in a concentrate loop. When the concentration of accumulated polyvalent ions becomes sufficiently large, the polyvalent ions with associated counterions can precipitate as scale on the side of a compartment separation membrane adjacent to a concentrating compartment and thereby foul the membrane. Furthermore, bacteria, over time, can grow in the concentrate loop and deposit on and foul the compartment separation membranes. In order to remove impurities from the concentrate loop, a bleed from the concentrate loop is required. Because fluid in the concentrate loop is continuously bled off, the fluid must be made up by additional fluid continuously provided to the concentrate loop. In U.S. Pat. No. 6,056,878 to Tessier et al., FIG. 3 illustrates that the reverse osmosis permeate is provided to the diluting compartments and is provided as make up water to the concentrate loop. The reverse osmosis membrane filters out polyvalent ions and bacteria; as a result, the use of the reverse osmosis permeate in the concentrate loop can reduce the rate of fouling of the compartment separation membranes from the rate if unfiltered supply water were used. However, the use of the reverse osmosis permeate requires a larger capacity reverse osmosis unit for a given volumetric rate flow of an EDI product stream than if the reverse osmosis unit permeate were not used as make up for the concentrate loop, resulting in greater complexity and capital costs.
The ratio of the flow rate of the EDI product stream to the flow rate of the supply stream can range between zero and one; the closer the ratio is to one, the more efficiently a water purification system uses water in the supply stream. A system providing reverse osmosis permeate to the diluting compartments and using reverse osmosis permeate as make up for a concentrate loop consumes more supply water in the supply stream per unit volume of purified water in the EDI product stream, than if the reverse osmosis permeate were not used as make up. That is, the ratio of the flow rate of the EDI product stream to the flow rate of the supply stream provided to the reverse osmosis unit is decreased from the ratio where reverse osmosis permeate is not used as make up. The system is therefore more expensive to operate and less environmentally friendly than if reverse osmosis permeate were not used as make up.
The recycle pump and the piping, valves, and controls associated with a concentrate loop add to the capital and maintenance costs of a water purification system and add to the bulk and weight of the system. The additional bulk and weight renders the system more difficult to transport and more difficult to install in confined spaces. The recycle pump increases the power required by the system to produce a unit volume of purified water in the EDI product stream.
An antiscalant agent can be injected into the concentrating feed stream to prevent or delay the precipitation of polyvalent ions and associated counterions as scale. An antiscalant agent injection device contributes to capital and maintenance costs and increases the bulk and weight of a water purification system. Similarly, an antibacterial agent can be injected into the concentrating feed stream, but the antibacterial agent must eventually be disposed of as waste, and an antibacterial injection device contributes to capital and maintenance costs and increases the bulk and weight of the system.
The reverse osmosis permeate has no more than a small concentration of ions, including monovalent ions. When the reverse osmosis permeate is added as make up to the concentrate loop to compensate for a continuous bleed, the concentration of ions in the concentrate loop, including the concentrating compartments, can be small. To maintain a large conductance across the concentrating compartments, a large concentration of ions in the concentrate loop must be maintained. To maintain a large concentration of ions, make up salt, e.g., monovalent salt, can be injected into the concentrate loop. In a monovalent salt, the ions which associate to form the salt are monovalent; sodium chloride is an example of a monovalent salt. In general, the concentration of monovalent salt in a concentrating compartment of an electrodeionization unit is such that monovalent salt is not deposited as scale; if monovalent salt does deposit as scale, it can be easily removed. However, a salt injection device has associated capital and maintenance costs and increases the bulk and weight of a water purification system. The salt added is eventually bled from the concentrate loop and must be disposed.
FIG. 5 of U.S. Pat. No. 6,056,878 to Tessier illustrates a system with two reverse osmosis units in series. As described in the patent document, the permeate stream from the first reverse osmosis unit is provided to the second reverse osmosis unit. The reject stream from the second reverse osmosis unit can be recycled back to the supply stream to the first reverse osmosis unit, discharged as waste, or provided as make up to the concentrate loop, in which fluid is cycled through the concentrating compartments of the electrodeionization unit. Although the concentration of ions in the reject stream from the second reverse osmosis unit may be greater than in the permeate stream from the first reverse osmosis unit, reverse osmosis membranes are good ion filters, and there will likely still be a need to inject additional salt into the concentrate loop. Furthermore, unless none of the reject stream from the second osmosis unit is discharged as waste, the ratio of the flow rate of the EDI product stream to the flow rate of the supply stream to the first reverse osmosis unit is lower than in the system illustrated in FIG. 3 of U.S. Pat. No. 6,056,878 to Tessier et al., in which a second reverse osmosis unit is not present. The addition of a second reverse osmosis unit increases the capital and maintenance costs of the system. In a system incorporating a concentrate loop, capital and maintenance costs are associated with the recycle pump and associated piping, valves, and controls. The recycle pump, piping, valves, and controls add to the weight and bulk of a water purification system, rendering it less portable and more difficult to install in confined spaces.
In an alternative approach, a concentrate loop is not used in a water purification system incorporating an electrodeionization unit. Instead, fluid in a concentrating feed stream is continuously provided to and passed only once through the concentrating compartments with no recycle of the fluid. Such a one pass concentrating feed stream system has several advantages over a system incorporating a concentrate loop. The one pass system is simpler than a system incorporating a concentrate loop in that the recycle pump, piping, valves, and controls associated with a concentrate loop are not required, so that the one pass system has lower associated capital and maintenance costs than a system incorporating a concentrate loop. Because fresh fluid in the concentrating feed stream is continuously provided to the concentrating compartments in a one pass system, polyvalent ions and bacteria do not accumulate, so that a one pass system can require less frequent cleaning of compartment separation membranes than a system incorporating a concentrate loop. Electropure, Inc. manufactures a one pass unit, the Electropure EDI.
However, a traditional one pass system that provides a portion of the reverse osmosis permeate stream to the diluting compartments and the remainder to the concentrating compartments of an electrodeionization unit is even more consumptive of water and has a lower ratio of EDI product stream flow rate to flow rate of the supply stream to the reverse osmosis unit than a system including a concentrate loop. The large rate of consumption of water contributes to the operating cost of a traditional one pass system. The required capacity and the associated capital cost of the reverse osmosis unit for a given EDI product stream flow rate can be greater than in a system incorporating a concentrate loop. Because ions driven from the diluting compartments into the concentrating compartments are not recycled to the concentrating compartments, and the permeate from the reverse osmosis unit has a small concentration of ions, there can be a need to inject salt into the concentrating feed stream of a traditional one pass system. Such injection of salt can be needed to ensure a large conductance across the concentrating compartments and ensure energy-efficient operation of the electrodeionization unit, i.e., an acceptable energy consumption per unit volume of purified water in the EDI product stream. For a given rate of flow of the EDI product stream, and a given composition of water in the supply stream, a traditional one pass system can require a greater rate of salt addition than a system incorporating a concentrate loop. The consumption of salt contributes to the operating cost and a salt injection device contributes to the capital and maintenance costs as well the bulk and weight of a traditional one pass system. The greater flow rate of supply stream water for a given flow rate of the EDI product stream in a traditional one pass system than in a system incorporating a concentrate loop can result in a traditional one pass water purification system being less environmentally friendly than a water purification system incorporating a concentrate loop.
There thus remains an unmet need for a water purification system that can operate for a long time before cleaning or replacement of membranes is required, is efficient in energy consumed per unit volume of purified water produced, is environmentally friendly in having a large ratio of the flow rate of the EDI product stream to the flow rate of the supply stream, is simple in design, has small capital and small maintenance costs, and is compact and has a small weight so as to be easy to transport and install.